People with attention deficit/hyperactivity disorder (ADHD) who take certain medications over long periods may develop changes in their brains that ultimately impact their ability to respond to the drugs, a new study suggests.

In the study, adults with ADHD had higher levels of a protein called a dopamine transporter in their brains after one year of treatment with the drug methylphenidate (most commonly sold as Ritalin), compared with before they starting taking the drug.

While some researchers have speculated that people with ADHD naturally have more dopamine transporters in their brains, the new study suggests that the high numbers are a result of treatment with medications, said study researcher Dr. Gene-Jack Wang, of Brookhaven National Laboratory in Upton, N.Y. Prior to the study, none of the participants had ever been treated with ADHD drugs.

Because the study only looked at patients over a one-year period, the consequence of this brain change is not known, Wang said.

But it's possible that the change could affect the way patients respond to the drugs.

Dopamine is a brain chemical that sends signals between nerve cells, and is linked with attention and pleasure. It's thought that people with ADHD have problems with their dopamine signaling, and drugs like Ritalin work by increasing levels of dopamine, which helps patients focus.

Dopamine transporters, on the other hand, clear dopamine after the chemical has sent a signal. Thus, more dopamine transporters could mean that dopamine gets cleared more quickly, particularly during times when patients stop taking their medications.

"This could result in more severe inattention and the need for higher doses of medication," the researchers write in the May 15 issue of the journal PLOS ONE.

Future research is needed to explore whether this increase in dopamine transporters does in fact lead to higher tolerance of ADHD drugs, Wang said. It's possible that, after patients stop taking the medications, the number of dopamine transporters is reduced.

Studies should also to determine whether some people are genetically prone to a lower tolerance of ADHD drugs, Wang said.

The study involved 18 adults with ADHD who had their brains scanned twice: once at the beginning of the study before any drug treatment, and once at the end of the study after one year of taking Ritalin.

Among these participants, there was a 24 percent increase in the number of dopamine transporters in some areas of the brain. In contrast, a group of healthy participants who did not take Ritalin had no increase in dopamine transporters after one year.